Neurological stimulators have been developed to treat pain, movement disorders, functional disorders, spasticity, cancer, cardiac disorders, and various other medical conditions. Implantable neurological stimulation systems generally have an implantable signal generator and one or more leads that deliver electrical pulses to neurological tissue or muscle tissue. For example, several neurological stimulation systems for spinal cord stimulation (SCS) have cylindrical leads that include a lead body with a circular cross-sectional shape and one or more conductive rings (i.e., contacts) spaced apart from each other at the distal end of the lead body. The conductive rings operate as individual electrodes and, in many cases, the SCS leads are implanted percutaneously through a needle inserted into the epidural space, with or without the assistance of a stylet.
Once implanted, the signal generator applies electrical pulses to the electrodes, which in turn modify the function of the patient's nervous system, such as by altering the patient's responsiveness to sensory stimuli and/or altering the patient's motor-circuit output. In SCS therapy for the treatment of pain, the signal generator applies electrical pulses to the spinal cord via the electrodes. In conventional SCS therapy, electrical pulses are used to generate sensations (known as paresthesia) that mask or otherwise alter the patient's sensation of pain. For example, in many cases, patients report paresthesia as a tingling sensation that is perceived as less uncomfortable than the underlying pain sensation.
Conventional implanted SCS pulse generators are typically charged using a set of fixed charging parameters. FIG. 1 illustrates a representative process 10 in accordance with the prior art for establishing and using battery charging parameters for standard implantable pulse generators. The process includes a pre-charge parameter selection process 11 and a charging process 12. The pre-charge parameter selection process 11 includes establishing fixed thresholds for charging parameters, including thresholds for charging voltage levels and/or charging current levels, and time limits for one or more phases of the charging process 12 (block 14). The charging process 12 itself includes an initial period during which the battery is charged at a constant current (block 15). At block 16, the process includes determining whether an end-of-charge voltage threshold has been reached. If the voltage threshold has not been reached, then the battery is further charged using the constant current value. If the voltage threshold has been reached, then in block 17, the battery is further charged at a constant voltage level, rather than a constant current level. In block 18, the charger determines whether a minimum end-of-charge current threshold has been reached or, alternatively, whether a charging time limit has been reached. If either condition is met, the charging process stops (block 19). If the relevant condition is not met, the constant voltage phase of the charging process continues until the current threshold or time limit has been reached.
In contrast to traditional or conventional (i.e., paresthesia-based) SCS, a form of paresthesia-free SCS has been developed that uses therapy signal parameters that treat the patient's sensation of pain without generating paresthesia or otherwise using paresthesia to mask the patient's sensation of pain. One of several advantages of paresthesia-free SCS therapy systems is that they eliminate the need for uncomfortable paresthesias, which many patients find objectionable. However, a challenge with paresthesia-free SCS therapy systems is that the signal may be delivered at frequencies, amplitudes, and/or pulse widths that use more power than conventional SCS systems. As a result, the battery of the implanted system can discharge and become depleted at an accelerated rate. Accordingly, a follow-on challenge with providing non-paresthesia-generating spinal cord stimulation via an implanted pulse generator is that, in at least some cases, it may be difficult to maintain an effective signal as the charge available from the pulse generator battery decreases. One approach to power consumption challenges in the context of conventional SCS systems is to increase the frequency with which the pulse generator is charged, but this can be inconvenient for the patient. Another approach is to add signal conditioning hardware, for example, to boost the voltage provided by the battery as the battery discharges. A drawback with this approach is that it can be inefficient. Accordingly, there remains a need for effective and efficient therapy signal delivery, despite the possibility of increased power consumption resulting from the signal delivery parameters used for paresthesia-free patient therapy.